A few notes: - the "stability" I refer to is always minor errors - I remove lots of packages every time I install it, although I've experienced system error notifications even when I didn't uninstall anything
All in all, I'd say that there is a lack of polishing, at the low-level, more than lack of stability.
There is no excuse for having the installation fail, though, and it happened a number of times.
To reply the question directly, I've used betas a few times, and they worked as much as the final version for me. I wouldn't do it now though - in the past, for my usage, some types of changes were very significant; today, I get very little in upgrading.